The present invention relates to a method for heat treating workpieces, such as coated turbine components, and to an improved system for performing the heat treat method of the present invention.
Overlay type metallic coatings (i.e. NiCoCrAlY, CoCrAlY, etc.) are mostly characterized by their oxidation resistant sub-alloy protection properties and improved life span within the turbine engine environment. These overlay metallic coatings may be applied to substrate surfaces by thermal spray processes, such as low pressure plasma spray and atmosphere pressure plasma spray, or by vapor deposition processes such as electron beam physical vapor deposition or cathodic arc. The density of the coating plays an important role in the oxidation resistance characteristics as well as the life span at which the coating will protect the substrate from the corrosive environment in which it operates. A coating free of open pockets, voids, fissures, cracks, or leaders provides significantly longer oxidation life protection than a coating containing such aforementioned characteristics. The state-of-the art technology used today to ensure that such coatings are close to 100% dense as possible is to apply the coating as dense as possible, then diffusion heat treat the coating, followed by subjecting the overlay coating to energy from processes such as peening. The peening process transfers enough kinetic energy at impact from the peen media velocity into the coating surface to increase the coating density by compaction and to improve the coating surface finish. The extent to which the peening process can improve the coating density and surface finish is related to the amount of kinetic energy that can be transferred from the peening media impact event onto and into the coating surface (often measured with almen strip intensity) in conjunction with the coating's ductility. It should be noted that to apply coatings which are excessively ductile will not provide the proper protection within the hot corrosive environments in which they operate. Also, if one applies a coating that is excessively hard, the coating will not react well to the peening process and will leave excessive porosity within the coating structure, ultimately resulting in a poor life oxidation resistance coating.